Organic light emitting displays are self-emission devices. Organic light emitting displays have higher luminescence than liquid crystal displays, and are thinner than liquid crystal displays, as organic light emitting displays do not include a backlight unit.
An organic light emitting display has a structure wherein an anode, an organic layer, and a cathode are sequentially stacked on a substrate on which a pixel circuit, such as a thin film transistor, is deposited. The structure of an organic light emitting display can be a top-emission structure or a bottom-emission structure. In a top-emission structure, an image is realized toward the opposite direction of the deposited substrate, i.e., toward the cathode, and thus the aperture ratio of the top-emission structure is higher than that of the bottom-emission structure where an image is realized toward the substrate. Accordingly, the light emitting efficiency of a top-emission organic light emitting device is higher than a bottom-emission organic light emitting device. However in the top-emission structure, the cathode must be transparent, which is difficult. Generally, a basic requirement of the cathode is that the cathode must have a lower work function than the anode, but a material having a low work function is generally a metal that has low light transmittance.
A conventional transparent cathode is created by forming a thin metal layer having a low work function, but the light transmittance is still very low, and it is difficult to improve light transmittance.
Due to this limitation, a micro-cavity that amplifies light emitted from an emission layer has been suggested. However, the optimum thicknesses of micro-cavities are different for each color, and thus the thickness of the organic layer disposed between the anode and the cathode are different for each color. In other words, in an organic light emitting display, light is emitted as excitons are formed in the organic emission layer as electrons are injected into the holes of the anode from the cathode. In order to adjust the distance from the excitons and the resonance thickness, the thickness of the organic layer, specifically the thicknesses of the hole or electron injection layer, or the hole or electron transport layer, must be varied. Because the thickness of the organic layer must be different for each color, independent masks are used for each color. However, such an independent depositing method is complex, and thus the production costs increase.
Also, to increase the resolution of a display, masks must have a higher resolution pattern, which is difficult to obtain in a large area display.
Moreover, the thickness of the organic layer cannot be determined based only on the optical efficiency, because when the thickness of hole layers or electron layers are different, electrical characteristics of the organic light emitting display may deteriorate.